The present invention relates generally to a vision testing apparatus and a method for testing vision in terms of contrast sensitivity and spatial frequency response. The apparatus includes light-polarizing optotypes which are useful in the testing of vision. The optotypes are viewed through a crossed-analyzer or light-polarizing filter. The light-polarizing characteristics of both the polarizing filter and the optotype is advantageously utilized in a method wherein the rotation of the filter causes the optotype to vary from a first condition, in which a pattern of the optotype is indistinguishable from its background, to a second condition in which the pattern is at a maximum level of contrast. During rotation of the filter, the patient will indicate a point at which he or she first detects the pattern of the optotype to thereby indicate the patient's contrast sensitivity to a particular pattern. The sequential presentation of a series of patterns having different spatial frequencies allows for the evaluation of a patient's spatial frequency response and for the further determination of a functional relationship between contrast sensitivity and spatial frequency response.
Spatial frequency and contrast sensitivity measuring devices are generally known in the art. For example, U.S. Pat. No. 4,365,873 describes a spatial frequency and contrast sensitivity test chart utilizing a multiplicity of grating patches which vary in their levels of relative contrast and also present differing spatial frequencies. The threshold levels of contrast sensitivity and spatial frequency response at which the presence of a grating can be detected accurately is reported to define a visual transfer function.
Such prior art devices and methods for using such devices have generally been limited to the use of charts such as those described in the aforementioned letters patent. The reliance on standard charts requires an extensive collection of gratings or patterns to fully evaluate the patient's so-called transfer function. Such charts generally fail to retain a consistent luminosity across the entire chart and from one grating or pattern to the next. Further, the use of charts presents a bias in the test procedure where the patterns or gratings are presented on a generally white background, such as is provided in the aforementioned Letters Patent. The use of white backgrounds for presenting a series of patterns or gratings is believed to bias the contrast sensitivity measurement in that the patient can view and compare the stark differences in contrast between the white background and the patterns thereon to obtain an additional clue during the determination of contrast sensitivity.
The present invention overcomes the aforementioned shortcomings, by providing an apparatus and a method for using the apparatus which includes means for presenting light-polarizing optotypes which may be viewed through a crossed analyzer or polarizer to determine contrast sensitivity and spatial frequency response for each of a series of such optotypes presented to the patient. By utilizing the light-polarizing qualities of the polarizer and the aforementioned light-polarizing optotype, thresholds of contrast sensitivity may be directly measured on a single optotype merely by rotation of the polarizer relative thereto or by rotation of the optotype relative to the polarizer. By rotation of the polarizer, the pattern of the optotype is slowly brought into higher contrast from an initial point wherein the pattern cannot be visually detected or distinguished from a background. The patient's sensitivity to spatial frequency may be tested by sequentially presenting a plurality of optotypes having different patterns which vary in their spatial frequencies to determine a threshold at which the patient is able to discern the presence of the pattern of that optotype from its background.
In a preferred embodiment, the apparatus of the present invention may be provided with a carousel capable of holding a plurality of optotypes to be presented individually to the patient. A single crossed analyzer or polarizer is provided in association with a light source and the carousel may be driven on command to position the individual optotypes in front of the light source and in association with the polarizer. The optotypes are prepared in a known manner on a transparent or translucent, oriented, linear polymeric material, such as oriented polyvinyl alcohol. The pattern of the optotype used to test spatial frequency response may be any pattern which is substantially uniformly repetitious. A plurality of optotypes having patterns of different spatial frequencies are positioned within the apparatus to facilitate testing.
In use, the apparatus can be used to measure contrast sensitivity by presenting a first optotype to be viewed by the patient through the polarizing filter. The polarizer is initially positioned with respect to the optotype to present the patient with a substantially uniformally transparent or translucent sheet on which no clear pattern is observable. Rotation of the analyzer, relative to the optotype, will gradually bring the pattern into view. While the analyzer is slowly rotated from its aforementioned initial position, the patient will indicate the point at which the pattern of the optotype can first be detected to indicate the patient's contrast sensitivity. An evaluation of spatial frequency response is accomplished by the sequential presentation of several optotypes to cover a range of spatial frequencies. The sequential presentation of a number of optotypes determines the patient's ability to detect or distinguish between the light and dark areas of the patterns, as presented, and thereby provides a measure of the patient's spatial frequency response. A transfer function for the patient is determined by sequentially presenting a number of optotypes to the patient until a low contrast level prevents the patient from detecting the pattern of the optotypes. The patient's threshold levels for contrast level and spatial frequency are quantified and compared with established norms.
It is accordingly an object of the present invention to provide an apparatus for measuring an individual's contrast sensitivity and spatial frequency response.
It is another object of the invention to provide an apparatus for evaluating spatial frequency and contrast sensitivity, the apparatus including means for presenting light-polarizing optotypes to a patient wherein a light polarizing filter is used so that the pattern of the optotype is selectively viewable by rotation of the filter.
It is still another object of the present invention to provide a method for evaluating spatial frequency and contrast sensitivity response which utilizes an apparatus including light-polarizing optotypes in conjunction with a polarizing filter.